Finding non-bias unclassified sources has proven to be a challenge. For example, there are Cuban Exile websites and extreme right wing sites that state beyond a doubt Cuba provided substantial logistical support. While it has been proven that Cuba provided support, the degree of support is hard to determine.

This first article is an excellent professional development article regarding logistic support for insurgents. A couple of excerpts below, much more at the link.

http://sci.tech-archive.net/Archive/...5-01/0005.html

The success of the Nicaraguan revolution in 1979
highlighted the need for a coordinated effort by all five anti-government groups operating in El Salvador. After months of negotiations in Havana, Cuba, the major guerilla groups emerged consolidated under the Unified Revolutionary Directorate (DRU). In part this unity is believed to have been due to the pressure of Fidel Castro who sought to consolidate training and
logistical support. Shortly after the announcement of the creation of
the DRU, in May, 1980, there were indications that approximately 600
tons of weapons had arrived in El Salvador.

The largest percentage of 5.56mm M16/AR15 rifles captured have been traced to weapons either provided to the Republic of South Vietnam or issued to US units sent to Vietnam. Both US Defense Intelligence Agency serial number trace data and the Institute's samples examined in El Salvador show the single largest source of M16/AR15 rifles provided to the FMLN has been
Vietnam.

Between 1984 and 1989, Eastern Europe emerged as an important new
source for FMLN small arms. Salvadoran government documents show that
prior to 1987 so-called "Communist Bloc" weapons represented less than
one percent of all captured weapons. By 1990 the total of such weapons
had grown to over 30 percent and continues to grow.

Eastern Europe has not been the only recent supplier of weapons to the
FMLN. Increasing numbers of North Korean manufactured 7.62 x 39mm Type
68 (AKM) rifles have been captured by government forces.

Logistical support for the FMLN has not been dependent upon a single
source, nor a single region.
This information seems plausable, but I'm a little leery of the source:

http://cuban-exile.com/doc_201-225/d....html#Salvador

With unified tactics and operations now possible, Cuba began to assist the guerrillas in formulating military strategy. Cuban specialists helped the DRU devise initial war plans in the summer of 1980. The Cubans influenced the guerrillas to launch a general offensive in January 1981. After the offensive failed, guerrilla leaders traveled to Havana in February 1981 to finalize a strategy to "improve our internal military situation" by engaging in a "negotiating maneuver" to gain time to regroup.(11)

Cuba provided few weapons and ammunition to Salvadoran guerrillas from its own resources but played a key role in coordinating the acquisition and delivery of arms from Vietnam, Ethiopia, and Eastern Europe through Nicaragua.(12) After the unmasking of this network, Cuba and Nicaragua reduced the flow in March and early April. Prior to a guerrilla offensive in August an upswing in deliveries occurred. The arms flow continues via clandestine surface and air routes. In addition, the Cubans over the past year have established a network of small ships to deliver arms to Salvadoran insurgent groups.

Cuba also assists the Salvadoran guerrillas in contacts with Arab radical states and movements to arrange military training and financing for arms acquisition. In September 1980, Cuba laundered $500,000 in Iraqi funds for the Salvadoran insurgents. In March 1981, the Salvadoran Communist Party Secretary General, Shafik Handal, visited Lebanon and Syria to meet with Palestine leaders. Cuba also coordinated the training of a relatively small number of Salvadoran guerrillas in Palestinian camps in the Mideast.
The El Salvador civil war still interests me, because of the second and third order effects ranging from MS-13 gangs (several members were in the FMLN), cached arms that are available to highest bidder, probably narcoterrorists, and the fact that the FMLN is now a credible political party that is capable of winning the national election in 2009. I don't know what color their stripes are now, but the effects of the conflict still linger.